1. Field of the Invention
The present invention relates to an automatic sound controlling apparatus outputting a signal with an opposite phase and an equivalent sound pressure to cancel the sound emitted from an engine etc. Particularly, this invention enables an adaptive filtering means used in the apparatus to follow the sharp attenation of a sound frequency and thereby improve the accuracy of producing the canceling sound.
2. Description of the Related Art
Conventionally, in order to reduce a sound produced from an engine etc., a passive canceling apparatus such as a muffler etc. has been used, but it is advantageous to improve said apparatus from a viewpoint of size, canceling characteristics etc. Conventionally, an active automatic sound controlling apparatus canceling a sound has output a compensating sound with an opposite phase and equivalent sound pressure from a sound source, but the frequency characteristics and stability etc. of this active automatic sound controlling apparatus per se are not sufficient, so realistic use thereof has diminished. But in recent years, as a result of expanding a frequency range to be treated owing to the development of a signal processing technique used in a digital circuit, many realistic automatic sound controlling apparatuses have been proposed (for example Japanese Unexamined Patent Publication 63-311396) such as a 2 microphones.cndot.1 speaker type active automatic sound controlling apparatus wherein a feedforward system detecting a sound by a microphone for a sound source provided at an upper stream of a duct and outputting a signal with an opponent phase and an equivalent sound pressure produced by a signal processing circuit from a speaker provided downstream of the duct, and a feedback system detecting a canceled result by a microphone for a cancelling point and feeding it back, are combined with each other. In addition an adaptive filter of this apparatus that forms the compensating sound such as a signal with an opposite phase and an equivalent sound pressure is made of a DSP (Digital Signal Processor).
On the other hand, in order to obtain a canceling effect with regard to a space with an sound source being unclear in position, for example, in a automobile room etc. it is necessary to provide a 1 microphone.cndot.1 speaker arrangement apparatus using only the feedback system without providing the microphone at the sound source.
In the above automatic sound controlling apparatus a 1 microphone.cndot.1 speaker set is provided as only the feedback of the prior art, however, in the case that the sound period of a sound source changes sharply, a problem arises in that the canceling effect is reduced since the signal delays more than the sound transfer characteristics at least from the microphone to the speaker, and is a defect of the feedback system.
Further in the case that the conventional automatic sound controlling apparatus is installed in automobiles etc. although it is difficult to provide a microphone to receive the sound as explained above, it is possible to form a sound simulating signal from an engine rotation number. However, a problem arises in that this signal is separate from a realistic signal although there is a quick response because of the signal of a feedforward system. On the other hand, a signal from a microphone as a feedback system, which is a difference signal between a muffler and a speaker, is added to a signal with an opposite phase and the equivalent sound pressure is obtained by said signal processing circuit so that it is possible to form a sound reproducing signal. However, a problem arises in that although there is the advantage that this signal is similar to a realistic signal, it is inferior to a response to a quick signal because it includes a delay characteristics. If either of the two is applied, it is difficult to improve the accuracy of the automatic sound controlling apparatus in all driving states of the automobile.